In recent lighting devices, in particular also in lighting systems for motor vehicles, light-emitting diodes (LEDs) are being used to an increased extent. Such LED lighting devices have many advantages, such as small dimensions, low power requirement etc., but, in contrast to conventional light-emitting means, they require a certain current firstly to achieve a certain brightness and secondly to emit a certain color. Therefore, it is conventional in LED lighting systems to adjust the light color via the current and the desired brightness via a pulse-width-modulated (PWM) power supply. For this purpose, in practice corresponding control devices, in particular with DC/DC converters, are known, wherein an output voltage which is subjected to closed-loop control can also be output by these DC/DC converters.
However, it is frequently also desirable to supply power to and drive a large number of LEDs, for example individually or interconnected in groups, wherein the LEDs can also be of different types. Such individual LEDs or LEDs interconnected in groups are referred to generally here as LED units.
In the case of a plurality of LED units, efficient and cost-effective driving is desirable, in which case it is necessary to deal with the problem of correcting different voltage values with superimposed interference, which can occur in an on-board power supply system of a motor vehicle, for example, in such a way that the LEDs do not produce any flickering light.
In practice, at present a dedicated controller is connected upstream of each LED or each group of LEDs, i.e. each LED unit, in order to subject the individual section currents for the respective LED units to closed-loop control. If, for example, five sections or five LED units are now provided, which is a conventional value in the context of lighting systems for motor vehicles, there is then the need to also provide five controllers for the individual five sections or LED units. Secondly, fluctuations and interference in supply voltage (on-board power supply system) are generally corrected via a DC/DC converter, whose output voltage is above its maximum output voltage. Such a converter with an increased output voltage is generally referred to as a boost converter, whereas a step-down DC/DC converter is generally referred to as a buck converter.
The boost converters used in practice for this purpose achieve a good level of efficiency given suitable dimensions. However, one problem consists in that, at a given current flowing through a plurality of LEDs, different voltage drops across the LEDs can be provided owing to differences in the LEDs. As mentioned, the current through the LEDs is also of significance for the light color to be emitted. For example, when a current of equal value is flowing through two series-connected LEDs, there may be a different voltage drop across each of these two LEDs. It is therefore necessary to provide a separate current at least for each section, for each LED unit.